Reverse current electrolytic process



March 31, 1964 B. w. NEHER 3,127,331

REVERSE CURRENT ELECTROLYTIC PROCESS Filed June l5, 1959 United States Patent 'Oiilice 3,127,331 Patented Mar. 31, 1964 3,127,331 REVERSE CURRENT ELECTROLYTIC PRCESS Byron W. Neher, Hudson, Wis., assigner to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed .lune 15, 1959, Ser. No. 820,254 6 Claims. (Cl. wrt- 18) This invention relates toa novel process for reproducing visible images. In one aspect, this invention relates to a method for the electrolytic destruction of the rectication effect of selected areas of photoconductive material bonded to an electrically conductive backing. ln another aspect, this invention relates to a novel process for reproducing visible images on a photosensitive sheet. Still `another aspect is the preparation of novel lithographic plates.

A method has been recently developed for developing permanent visible images on suitable photoconductive copy papers which have been exposed to light images. The method involves the rapid continuous electrolysis of an electrolytic developer solution, and particularly, the electrodeposition of a metallic or other visibly distinct coating, at the exposed photosensitive surface. No preliminary electrostatic charging of fthe copysheet is required, and the copy produced needs no further heating or other processing to render the image permanent. Briefly, the copy sheet comprises a photoconductive powder, such as Zinc oxide, bonded to a contiguous electrically conductive backing. Since the photoconductive coating shows a rectication eliect, i.e. allows current to pass essentially in only one direction, it is more eiective to use the electrically conductive backing sheet `as the cathode in the electrolytic development process. With the backing sheet as anode, the electrical resistance of the photoconductive material is high, even in wet light struck areas.

Because of the rectiiication eiiect of photoconductors such as Zinc oxide, on a conductive backing sheet, the electrolytic development of the light struck photoconductive areas has been generally restricted to the use ot those materials which are reduced at the cathode to produce a visible image. Many oxidizaole developers and materials bearing a negative charge have not heretofore been efficiently useable in the above-described process.

`lt is therefore an object of this invention to provide -a reverse current process for the development of an exposed photocondnctive image.

It is another object of this invention to provide a process for developing photoconductive images by electrolytic oxidation.

lt is a further object of this invention to provide an electrolytic method for preparing lithographie plates.

Still another object `of this invention is to provide novel lithographic plates.

In accordance with this invention, a photosensitive sheet is m-ade of photoconductive material, usually in powdered form, bonded to an electrically `conductive backing, preferably by means of a suitable nonconductive binder material. After exposure of the photoconductive layer to a light image, the electrically conductive back- `ing of the photosensitive sheet is connected to the negative pole ot' a direct current source and electrolyzed in the presence of a conductive salt of a reducible metal. During electrolysis, the metal of the conductive salt is plated out as a conductive metallic lay'er or coating on the light struck areas of the photoconductive surface. If this electrolysis is carried out with the electrically conductive backing, connected as the anode, i.e. if the electrodes are reversed, the rectification effect or select-ive resistance of the photoconductive layer greatly hinders and substantially prevents free current passage in the opposite direction. However, it has now been rfound that the presence of free metal, which is plated out on the photoconductive surface in a differential pattern corresponding to the original light image, unexpectedly destroys the rectification elect of the photosensitive sheet in those lareas containing the free metal and permits current passage in both directions. Upon reversing the electrodes and making the electrically conductive backing the anode, the conductive image can be developed relectrolytically by an anodic process or reverse current process.

Among the developers which may be used in this anodic process are substances capable of changing color value on oxidation, such as the leuco form of vat dyestuls used in the dyeing of various commercial bers.. For example, if the anodic process is carried out with Indigo White in contact with the metal plated photoconductive surfaces, the `anodic reaction oxidizes Indigo White from its colorless leuco form to insoluble colored Indigo in the conductive surface areas. The final visible image is found to be stable except for the tendency to fade slowly, probably because of the oxidation of the leuco dye on exposure to air. These dyestuis can be incorporated into the electrolyte or may be coated on metal plated photoconductive surface prior to electrolysis.

Still another developer material that may oe employed in the anodic development process is the colored anion, as exemplified by the 4acid type dyestuffs. By carrying out the electrolysis with the metal plated photosensitive sheet as anode and with an acid type dyestul in the electrolyte, the colored anions of the acid type dye migrate selectively to the conductive metal plated image areas and are deposited thereon, thereby coloring the light exposed and metal plated surface areas. These dyes are commonly marketed in the form of a salt of their sulphonic acid, usually the sodium salt. Illustrative of such developers are the nitro dyestuis, such as Naphthol Yellow (C.I.9), the mono-azo dyestnffs, such as Fast Red (C1176), the dis-azodyestutis, such as Crocein Scarlet (C1277), the nitro-dyestufis, such as Naphthol Green (CLS), the triphenylmethane dyestutls, such as Wool Green (C.i.737), the xanthene dyestulis, such as Erio Fast Fuchsine BL (01.758), the orthraquinone dyestuts, such as Solway Blue SES (61.1053), the azine dyestuiis, such Vas Azocarmine (C1828) and the quinoline dyestufs, such as Quinoline Yellow (C.l.8G\l). Although some color is often deposited in the background areas when the colored anion containing electrolyte is brought into contact with the metal plated photosensitive sheet surface, the depth of color is significantly greater in the light struck, metal plated areas, and the contrast can be controlled by selection of the colored anion, concentration of colored anion in the electrolyte, duration and conditions of the electrolysis, etc.

In some cases, during the above reverse current development some or the metal plated on the light struck areas may be Ireoxidized by the anode reaction, with a resultant decrease in current ow. However, this does not significantly affect the color development, and for certain purposes is desirable in order to improve the quality' or intensity of the linal color,

lt is also within the scope of this invention to incorporate colored or uncolored negatively charged particles into the electrolyte and carry out electrolysis using the metal plated photosensitive sheet as anode, as earlier described. These negatively charged particles are preferably in suspension or dispersion and may constitute a latex, such as a latex of polyethylene, polypropylene, etc., or a hydrosol of such materials as Aniline Blue, Indigo, etc. Generally, those polymer latices stable in alkaline media contain negatively charged particles and are therefore operable in the instant electrolysis. The charge on s such particles is readily determined by well known methods.

During the electrolysis the negatively charged particles are deposited selectively on the metal plated or conductive areas of the photoconductive layer. When such negatively charged particles constitute materials which are hydrophobic in relation to the photoconductive surface and are therefore selectively ink receptive and water rejecting, e.g. polyethylene, polypropylene, etc., the surface of the resulting polymer coated sheet displays ink receptive, hydrophobic properties in the polymer coated, light struck areas and relative ink rejecting, hydrophilic properties in the background areas. Other negatively charged particles or ions which develop a layer or lm on the light struck image areas that display ink receptive properties different from, i.e. relative to, the background or non-light struck areas, would also produce a similar result. The selectively coated photosensitive sheet having varying degrees of ink receptivity, corresponding to the original light image, provides a simple, inexpensive and effective lithographie plate and produces outstanding reproductions when used in conventional lithographie processes, e.g. the Multilith process. ln certain instances it may also be desirable to incorporate certain ller materials in the negatively charged particles, thereby to alter the physical properties of the deposited polymer layer. These fillers need not carry an electrical charge, since the negatively charged particles of polymer serve as carrier for these iiller materials. Suitable llers include colored pigments, carbon black, silica, etc.

FIGURE 1 illustrates the above-described novel lithographic sheet obtained in accordance with this invention.

In the above process, the deposition of free metal can be effected by electrolysis of an organic or inorganic conducting salt of a reducible metal, such as silver nitrate, nickelous chloride, copper sulfate, etc. Any salt of an electrolytically reducible metal, which metal is electrically conductive, may be used. Moreover, the unreduced metal salt can be incorporated in or on the photoconductive material and thereafter be reduced in situ by electrolysis. The actual mechanism by which this deposited free metal destroys the rectification effect of the photoconductive material is not fully understood.

The receptor sheet which is exposed to the light image and on which the reduced metal is differentially deposited contains an electrically conductive base, such as metal foil, upon which a photoconductive layer, having a photo-conductivity value of at least about "7 mho/cm. and a dark conductivity value not greater than about one-twentieth of the photoconductivity value, is placed or bonded. Such sheets are described in greater detail in SN 692,529, led October 28, 1957, now U.S. Patent No. 3,010,884. In bonding the photoconductive material to the electrically conductive base, various Water resistant, flexible adherent film forming polymers can be used, provided the polymer is light in color and does not adversely affect the light sensitivity of the photoconductive material. Such polymers include, for example, a 30:70 copolymer of butadiene and styrene (Pliolite S-7 solution, 30% solution in toluene), polystyrene, chlorinated rubber, rubber hydrochloride, polyvinylidene chloride, nitrocellulose, polyvinyl butyral, silicones (e.g., D-C 803 silicone solution, 50% solution in xylol of alkyl amyl silicone resin capable of curing one hour at 480 F. to a hard and somewhat brittle polymer), etc. Polymers which are dissolved or softened by water, or which are dark in color, or insoluble in commercial solvents, or reactive with the photoconductive material, or which readily wet the photoconductive particles, are found to be less desirable. Thus, polyvinyl alcohol, polyacrylic acid, shellac and sodium carboxymethyl cellulose are generally not employed as binders for the light-sensitive materials. Ratios of binder to photoconductive powder generally range from 1:10 to 1:1, preferably about 1:5 to 1:2.

As light-sensitive materials, zinc oxide, cadmium sulde, zinc sulfide, and other photoconductive powders having photoconductivity values as above described, have also been found to provide adequate photoconductivity values in coated lm form and to produce receptor sheets suitable for use in the instant invention. Mixtures of these photoconductive materials may also be employed. Generally, Zinc oxide is preferred. To improve or enhance the sensitivity of these photoconductive materials in certain visible and non-visible areas of the light spectrum, dye sensitizers such as Acridine Orange, Fluorescein, Eosin Y, Rose Bengal, Methylene Blue, etc., are preferably admixed in small quantities with the photoconductive powder.

Metal foil or sheet provides a suitable electrolytcally conductive backing. Metal conductors, such as aluminum, chromium, nickel and copper, are suitable for the electrically conductive backing and may additionally be placed on the surface of a nonconductive supporting sheet, e.g., by vapor deposition, lamination, etc. When the photosensitive sheet is immersed in the electrolyte during electrolysis the exposed conductive backing must be insulated from the electrolyte, e.g. by a nonconductive surface coating such as plastic, etc.

The following examples are illustrative and are not to be construed as limiting the scope of the instant invention.

Example 1 A suitable light-sensitive sheet material was lirst prepared. A flexible ilm of transparent cellulose acetate having a thickness of about 10 mils (0.010 inch) was first metallized on one surface, by vapor deposition in a vacuum, with an extremely thin coating of aluminum. The coating was found to have a surface resistivity of about 200 ohms per square, and transmitted about 55% of incident light in the visible range. Over this metal layer Was then applied a suspension of 48 parts by weight of Merck & Companys Reagent Grade Zinc oxide microcrystals in a solution in 48 parts toluene, of 4 parts of pliolite S-7 resin, a resinous copolymer of about 30 parts butadiene and correspondingly about parts styrene, serving as a binder, the mixture having been ground in a ball mill until smooth. After drying, the irmly bonded smooth white coating was found to be between 0.3 and 0.6 mil in thickness. The sheet material was highly water-resistant.

Sheet material prepared as just described was suspended in a transparent glass cell containing a solution of 28 grams of copper sulfate in 200 ml. of water. A at electrode of slightly larger area, in this case a copper plate, was suspended in the solution facing and somewhat removed from the coated surface of the sheet material. A light-image was focused on the uncoated surface of the sheet through the glass wall of the cell, the source of the light being a 10G-watt bulb and providing an intensity of about 70 foot-lamberts. Exposure was maintained for about 5 seconds. A source of potential was then connected across the copper plate and the conductive aluminum layer of the sensitive sheet, the latter being connected to the negative pole, and a current of about 15 milliamperes was passed through the system for about 3 seconds. The sheet was withdrawn and rinsed, and was found to have a negative reproduction of the light-image on the sensitive coating. Non-illuminated areas of the sensitive coating remained White, while the exposed areas were darkened by deposition of metallic copper thereon.

Equally effective copy was obtained by exposing the coated sheet to the light-image under dry conditions, and then promptly immersing the sheet in the electrolytic cell and electrolytically developing the image in the manner described.

Silver nitrate solution was substituted for the copper sulfate to provide equally effective image development. Nickelous chloride is also effective, and is improved by the addition of sodium thiosulfate. A particularly eifective developing solution contains nickelous chloride and 5% sodium thiosulfate.

The ratio of pigment to binder in the light-sensitive coating was eifectively varied over wide ranges. At 12 parts of zinc oxide to one of resin, as in the specific formula just given, the white areas of the print are sometimes found to contain dark spots, indicating non-uniform or insuflicient resistivity. Excellent prints are obtained at lower ratios, for example at 8:1 and at 4:1. Somewhat less effective prints are obtained at 3 :1 ratios of zinc oxide and Pliolite resin, and at 2:1 the light-sensitivity is inadequate and the results are decidedly inferior. These ratios may be specifically different with other specific oxides and resins but will serve to illustrate a generally desirable range.

Electrically conductive glass plates have been substituted for the partially transparent metallized cellulose acetate film as a carrier or base for the light-sensitive coating, A glass having a surface layer high in stannic oxide, having a surface resistivity of about 600 ohms per square and a light transmission of at least about 90%, has proven useful, although somewhat lower resistivity is preferred.

The sensitive surface of such transparent photosensitive coated plates is effectively exposed to the light-image through the transparent plate and simultaneously electrolytically developed, as described in the foregoing example. These plates may alternatively be rst exposed to the light-image and then, without further irradiation, transferred to the developing station and separately developed, the light-memory of the zinc oxide coating being suliicient to maintain the necessary conductivity at the irradiated areas. The latter procedure is equally effective on fully opaque plates such as metal plates coated with the sensitive zinc oxide coating.

Opaque plates have been simultaneously exposed and developed by substituting a copper wire frame for the copper plate of Example l and then exposing the sensitive surface of a coated metal plate to a light-image through the frame while carrying out the electrolysis as before. Where the plate area is too large for uniform electrolysis in this manner, a screen is provided in place of the frame, and the screen is moved steadily during electrolysis so as to avoid producing a visible shadow pattern on the sensitive sheet.

Example 2 A photosensitive sheet having a 5 mil conductive aluminum backing and a coating thereon of zinc oxide and Pliolite 3 7 resin (about 10:1 respective weight ratio) was exposed to a light image for a period of 15 seconds, using a 100 watt light source to project the image from a positive microfilm frame to the photoconductive surface of the sheet. The exposed surface was then brought into contact with a solution of 10% by weight of nickelous chloride and 5% by weight of sodium thiosulfate and electrolytically developed at an impressed potential of between about 30-40 volts D.C., the conductive aluminum backing being connected to the negative pole of the current source and being masked on its exposed face with a plastic covering, as described in Example l. After a period of l5 seconds the electrolysis was stopped and the exposed surface was rinsed with cold water to remove residual metal salts. A negative reproduction of the light image was formed, the nickel having been reduced and deposited as the free metal on the light struck areas of the photosensitive surface. Conversely, if a negative light-image is used, a positive reproduction is formed.

The sheet was then suspended in a 1% solids aqueous polyethylene latex [A-C Polyethylene 629, Allied Chemical and Dye Company, M.P. Z13-221 F., penetration (l00 gm., 5 sec., 77 F.) of 3-6, acid No. 14-17, color less than l NPA]. Using the conductive aluminum backing sheet as the positive pole and masking the exposed aluminum backing with a plastic covering, the polyethylene latex was electrolyzed at a potential of about 30-40 volts for about 15 seconds. Polyethylene was deposited selectively on those light struck areas having a free metal deposit, producing a polyethylene image which had greater ink receptivity relative to the zinc oxide background areas. When used as a lithographic plate negative prints were obtained. If the original light-image is negative, the final lithographie prints are positive.

Other polymeric latices which are stable in alkaline media and which contain negatively charged particles can be similarly employed, including polytetratiuoroethylene, snythetic rubber, eg. Chemigum latex 24S-B (butadieneacrylonitrile, non-staining, oil resistant, vulcanizable synthetic rubber latex supplied by the Goodyear Tire and Rubber Company), polyvinyl acetate, polystyrene, Pliolite, rubber, polyvinylidene chloride (Saran), etc.

The above described lithographie plate was installed on a Multilith lithographie machine and provided over 250 copies with excellent contrast and definition. An acid fountain solution containing carboxymethyl cellulose and various inks were used. Final prints were dried at 200 F. for l5 minutes.

Example 3 A nickel coated photosensitive sheet was prepared as described in Example 2. Using the conductive aluminum backing as the positive pole, a sponge containing a dilute aqueous solution of p-amino-diethyl-aniline, p-nitro-benzyl cyanide, and sodium sulfite, was connected to the negative pole of the current source and was slowly drawn over the differentially metal coated surface. A potential of 30-40 volts was used. A magenta colored image was thereby electrolytically developed, the p-amino-diethylaniline developer being oxidized and coupling with the p-nitro-benzyl cyanide to produce the magenta color on those surface areas containing the electrolytically deposited metal. The metal deposit was slowly removed by oxidation during the electrolytic reaction, producing an excellent magenta negative image.

When alpha naphthol and ethyl aceto-acetate were substituted as couplers, the resulting images were cyan and yellow respectively. Other oxidizable developers, such as phenolic compounds, eg. pyrogallic acid, etc., can be used in place of p-amino-diethyl-aniline.

Various other embodiments and variations within the scope of this invention will be readily apparent to persons skilled in the art.

Having thus described my invention, l claim:

l. A process for reversing current tlow through light struck areas of a photoconductive layer which comprises electrolyzing a conductive solution containing ions of a ietal capable of being electrolytically reduced to the free metal state, the cathode being an electrically conductive sheet containing said photoconductive layer bonded to one electrically conductive contiguous surface thereof, thereby selectively depositing free metal on those light struck portions of the photoconductive layer and differentially destroying the rectication properties of the light struck portions of the photoconductive layer, and subsequently carrying out another electrolytic reaction with said electrically conductive sheet as anode.

2. A process for developing an exposed photoconductive sheet which comprises electrolyzing a conductive solution containing ions of a metal capable of being electrolytically reduced to the free metal state, the cathode being an electrically conductive sheet containing a photoconductive layer bonded to one electrically conductive contiguous surface thereof, thereby selectively depositing free metal on those light struck portions of the photoconductive layer and differentially destroying the rectification properties of said portions of the photoconductive layer, and subsequently carrying out another electrolytic reaction in which said sheet is the anode and is in contact with a material which changes color value upon oxidation.

3. The process of claim 2 in which the material which 7 ,Y changes color value upon oxidation is a leuco vat dyestuff.

4. A process for reproducing a light image on a novel exposed lithographie sheet which comprises electrolyzing a conductive solution containing ions of a metal capable of being electrolytieally reduced to the free metal state, the cathode being an electrically conductive sheet containing a photoconductive layer bonded to one contiguous electrically conductive surface thereof, thereby selectively depositing free metal on those light struck portions of the photoconductive layer and dilferentially destroying the rectification properties of said portions of the photoconduetive layer, and subsequently eleetrolyzing a polymeric latex containing negatively charged polymer particles in contact with said electrically conductive sheet as anode.

5. A process for developing an exposed photoconductive sheet which comprises electrolyzing a conductive solution containing ions of a metal capable of being electrolytically reduced to the free metal state, the cathode being an electrically conductive sheet containing a photoconductive layer bonded to one contiguous electrically conductive surface thereof, thereby selectively depositing free metal v ,Y 8 on those light struck portions of the photoconductive layer and differentially destroying the rectification properties of said portions of the photoconductive layer, and subsequently electrolyzing a solution containing a colored anion using the above electrically conductive sheet as anode.

6. The process of clairn 5 in which the colored anion is the anion of an acid type dyestuff.

References Cited in the tile of this patent UNITED STATES PATENTS 

1. A PROCESS FOR REVERSING CURRENT FLOW THROUGH LIGHT STRUCK AREAS OF A PHOTOCONDUCTIVE LAYER WHICH COMPRISES ELECTROLYZING A CONDUCTIVE SOLUTION CONTAINING IONS OF A METAL CAPABLE OF BEING ELECTROLYTICALLY REDUCED TO THE FREE METAL STATE, THE CATHODE BEING AN ELECTRICALLY CONDUCTIVE SHEET CONTAINING SAID PHOTOCONDUCTIVE LAYER BONDED TO ONE ELECTRICALLY CONDUCTIVE CONTIGUOUS SURFACE THEREOF, THEREBY SELECTIVELY DEPOSITING FREE METAL ON THOSE LIGHT STRUCK PORTIONS OF THE PHOTOCONDUCTIVE LAYER AND DIFFERENTIALLY DESTROYING THE RECTIFICATION PROPERTIES OF THE LIGHT STRUCK PORTIONS OF THE PHOTOCONDUCTIVE LAYER, AND SUBSEQUENTLY CARRYING OUT ANOTHER ELECTROLYTIC REACTION WITH SAID ELECTRICALLY CONDUCTIVE SHEET AS ANODE. 